1. Field of the Invention
The present invention relates to a liquid fuel cartridge which stores water generated by a fuel reaction, and a direct liquid feed fuel cell having the same.
2. Discussion of the Background
A direct liquid feed fuel cell is an electric generator which generates electricity by an electrochemical reaction between an organic compound fuel such as methanol or ethanol, and an oxidant such as oxygen. Since a direct liquid feed fuel cell has very high energy density and power density and directly uses a liquid fuel such as methanol or ethanol, a peripheral device such as a fuel reformer is not needed and fuel can be easily stored and supplied.
Referring to FIG. 1, a conventional direct liquid feed fuel cell has a structure in which an electrolytic membrane 1 is interposed between an anode 2 and a cathode 3. The anode 2 and the cathode 3 respectively include diffusion layers 22 and 32 for supply and diffusion of fuel, catalyst layers 21 and 31 in which an oxidation/reduction reaction of fuel occurs, and electrode supporters 23 and 33. A precious metal catalyst such as platinum with excellent work function even at a low temperature can be used for a catalyst for the electrode reaction. An alloy containing a transition metal such as ruthenium, rhodium, osmium, or nickel can be used to prevent catalyst poisoning caused by carbon monoxide, a reaction by-product. Carbon-based material such as carbon paper or carbon fabric, is frequently used for the electrode supporters 23 and 33, and may be water-proof to maintain flow channels so that fuel can be supplied and a reaction product can be discharged. The electrolytic membrane 1 may be a high molecular membrane having a thickness of 50-200 μm, and may be a hydrogen ion exchange membrane which contains moisture and has ion conductivity.
An electrode reaction of a direct methanol fuel cell (DMFC), which uses methanol and water as mixed fuel, includes an anode reaction in which fuel is oxidized, and a cathode reaction caused by reduction of hydrogen ions and oxygen. The reactions are presented in equations 1, 2, and 3.CH3OH+H2O→CO2+6H++6e− (Anode reaction)  (1) 3/2O2+6H++6e−→3H2O (Cathode reaction)  (2)CH3OH+ 3/2O2→2H2O+CO2 (Overall reaction)  (3)
In the anode 2, where oxidation occurs according to equation 1, carbon dioxide, hydrogen ions, and electrons are generated by a reaction between methanol and water. The generated hydrogen ions are transferred to the cathode 3 via the electrolytic membrane 1. In the cathode, where reduction occurs according to equation 2, water is generated by a reaction between hydrogen ions, electrons supplied via an external circuit (not shown) and oxygen. Thus, in a DMFC overall reaction according to equation 3, methanol and oxygen react to form water and carbon dioxide. According to the molar ratio of equation 3, 1 mole methanol reacts with oxygen to form 2 moles of water.
Since methanol as liquid fuel reacts with water at a molar ratio of 1:1, as shown in equation 1, a mixed liquid with 1 mole methanol and 1 mole water can be used as a liquid fuel. This mixed liquid is about 64% methanol, by weight. However, when using high-concentration fuel with a 1-to-1 ratio of methanol to water, electric generating performance may be degraded due to crossover of fuel across the ion exchange membrane before the fuel can react with the catalyst in the anode. Thus, to avoid crossover, dilute low-concentration methanol with between 2 and 8% by volume can be used. However, when using low-concentration methanol, the total quantity of methanol is low, and thus the total amount of energy from the low quantity of fuel is accordingly reduced. Thus, in order to increase the amount of energy from the fuel, a fuel cell system with a fuel tank able to store a high-concentration of pure methanol is required.
Further, processing water generated by a reaction in a fuel cell is problematic. The water may be stored in a separate water tank in a fuel cell system.
U.S. Pat. No. 6,303,244 discloses a technique for storing methanol and water separately, mixing them using a mixer, and supplying them to a fuel cell stack, as shown in FIG. 2.
Referring to FIG. 2, air for a reduction reaction is supplied to an internal cathode of a stack 4 and vented to the outside from the cathode. Thus, water as a chemical by-product is withdrawn and drained into a water tank 6. High-concentration or pure methanol is stored in a fuel tank 7.
Water and methanol are stored in separate tanks 6 and 7, and water and methanol are each pumped to a fuel mixer 8, mixed at the fuel mixer 8 and supplied to an anode of the stack 4.
However, using this method, a separate water tank 8 is needed in addition to fuel tank 7. Furthermore, a pump is needed for each tank. Due to the space occupied by the separate water tank and the pumps, the energy density of the fuel cell system is reduced, thereby reducing the inherent advantages of a DMFC.